Node editor: when adding a new node, automatic links are created:

- from all selected nodes
- only connections between highest order socket types; so if there's RGBA
  and Value sockets, only RGBA sockets are connected. This because in
  these cases the Value sockets usually are for user input.
  Example: Mix node.

Thanks Trip for the hint!

source/blender/include/BSE_node.h

@@ -79,6 +79,8 @@ int node_has_hidden_sockets(struct bNode *node);
 struct bNode *node_add_node(struct SpaceNode *snode, int type, float locx, float locy);
 void node_adduplicate(struct SpaceNode *snode);
 
+void snode_autoconnect(struct SpaceNode *snode, struct bNode *node_to, int flag);
+
 /* ************* drawnode.c *************** */
 struct SpaceNode;
 struct bNodeLink;

source/blender/makesdna/DNA_node_types.h

@@ -140,6 +140,8 @@ typedef struct bNode {
 #define NODE_ACTIVE_ID		32
 #define NODE_DO_OUTPUT		64
 #define NODE_GROUP_EDIT		128
+		/* free test flag, undefined */
+#define NODE_TEST			256
 
 typedef struct bNodeLink {
 	struct bNodeLink *next, *prev;

source/blender/src/editnode.c

@@ -1380,6 +1380,58 @@ void node_border_select(SpaceNode *snode)
 
 /* ****************** Add *********************** */
 
+void snode_autoconnect(SpaceNode *snode, bNode *node_to, int flag)
+{
+	bNodeSocket *sock, *sockfrom[8];
+	bNode *node, *nodefrom[8];
+	int totsock= 0, socktype=0;
+
+	if(node_to->inputs.first==NULL)
+		return;
+	
+	/* no inputs for node allowed (code it) */
+
+	/* connect first 1 socket type now */
+	for(sock= node_to->inputs.first; sock; sock= sock->next)
+		if(socktype<sock->type)
+			socktype= sock->type;
+
+	
+	/* find potential sockets, max 8 should work */
+	for(node= snode->edittree->nodes.first; node; node= node->next) {
+		if((node->flag & flag) && node!=node_to) {
+			for(sock= node->outputs.first; sock; sock= sock->next) {
+				if(!(sock->flag & (SOCK_HIDDEN|SOCK_UNAVAIL))) {
+					sockfrom[totsock]= sock;
+					nodefrom[totsock]= node;
+					totsock++;
+					if(totsock>7)
+						break;
+				}
+			}
+		}
+		if(totsock>7)
+			break;
+	}
+
+	/* now just get matching socket types and create links */
+	for(sock= node_to->inputs.first; sock; sock= sock->next) {
+		int a;
+		
+		for(a=0; a<totsock; a++) {
+			if(sockfrom[a]) {
+				if(sock->type==sockfrom[a]->type && sock->type==socktype) {
+					nodeAddLink(snode->edittree, nodefrom[a], sockfrom[a], node_to, sock);
+					sockfrom[a]= NULL;
+					break;
+				}
+			}
+		}
+	}
+	
+	ntreeSolveOrder(snode->edittree);
+}
+
 /* can be called from menus too, but they should do own undopush and redraws */
 bNode *node_add_node(SpaceNode *snode, int type, float locx, float locy)
 {

source/blender/src/header_node.c

@@ -198,12 +198,22 @@ static uiBlock *node_selectmenu(void *arg_unused)
 void do_node_addmenu(void *arg, int event)
 {
 	SpaceNode *snode= curarea->spacedata.first;
+	bNode *node;
 	float locx, locy;
 	short mval[2];
 	
+	/* store selection in temp test flag */
+	for(node= snode->edittree->nodes.first; node; node= node->next) {
+		if(node->flag & NODE_SELECT) node->flag |= NODE_TEST;
+		else node->flag &= ~NODE_TEST;
+	}
+	
 	getmouseco_areawin(mval);
 	areamouseco_to_ipoco(G.v2d, mval, &locx, &locy);
-	node_add_node(snode, event, locx, locy);
+	node= node_add_node(snode, event, locx, locy);
+	
+	/* uses test flag */
+	snode_autoconnect(snode, node, NODE_TEST);
 		
 	addqueue(curarea->win, UI_BUT_EVENT, B_NODE_TREE_EXEC);